SALA PENAL PERMANENTE
SEXTO. ANÁLISIS DEL CASO
7.5. De lo señalado, se advierte claramente que los procesados brindaron en
Frame Relay is a connection-oriented, packet-switching protocol used to pass information across a digital interface. “Connection-oriented” means that a connection between the communicating parties is set up before the actual data transfer takes place. Other higher layer protocols are required to handle additional network services, such as flow control and error recovery. This protocol operates in the lowest sublayer of the data link layer (DLL) of the Open System Interconnection (OSI) reference model.
Detailed information on the Frame Relay protocol can be found in the following documentation:
• ANSI T1.606, 1990, Frame Relay Bearer Service.
• ANSI T1.617, 1991, Signaling Specification for Frame Relay Bearer Service.
• ANSI T1.618, 1991, Core Aspects of Frame Protocol for Use with Frame
Relay Bearer Service.
• ANSI T1.606a, 1992, Frame Relay Bearer Service - Congestion
Management and Frame Size.
• ANSI T1.513, 1994, User Information Transfer Performance Parameters.
These documents can be obtained from the American National Standards Institute.
Additionally, RFC-1490 has been used as a reference for the multiprotocol interconnection over Frame Relay.
The following is a list of additional ANSI standards available on the Frame Relay protocol:
• ANSI T1.606b, 1993, Frame Relay Bearer Service - Network-to-Network
Interface Requirements.
• ANSI T1.617a, 1994, Signaling Specification for Frame Relay Bearer Service
Reference Guide 1—Frame Relay
Model Features
This section discusses the implementation choices made in developing this OPNET Frame Relay model. Certain parts of the protocol have been simplified or omitted in view of the fact that it is intended primarily for performance estimation.
The following information is important in order to determine whether the OPNET Frame Relay example model is applicable for a particular simulation study or not.
The main limitations of the described model are:
• The model does not incorporate the SVC implementation for call
establishment; rather all the virtual circuits used for data transfer are modeled using the PVC approach. The PVC approach assumes that the user sets up the virtual circuits at the service subscription time and these virtual circuits remain intact during the length of the simulation. Specific instructions for setting up a PVC connection between source-destination application pairs are provided later in this document.
• The Local Management Interface (LMI) message passing is not
implemented. The LMI messages, if implemented, help in providing the user with status and configuration information for the PVCs operating over the Frame Relay interface. This feature is not implemented based on the assumption that once the PVC circuits are setup, they are not released until the end of the simulation.
• The Consolidated Link Layer Management (CLLM) message provides a
useful feature to the Frame Relay protocol by acting as an additional congestion notification mechanism. However, since this feature is optional and many systems do not implement this feature, the current model does not support it.
• The frame check sequence (FCS) checking is not implemented. This feature
of the Frame Relay protocol is used to ensure that transmission errors get trapped by the receiving stations. However, assuming that transmission errors are very rare, the implementation of this feature has been omitted from the simulation model.
• The model provides only a store-and-forward switching methodology (i.e.,
the frame switching takes place within a network switching device only after it is completely received). There may be instances when the frame switching equipment employs a cut-through switching technology (i.e., the leading edge of the incoming frame may be forwarded on to the next link before the
1—Frame Relay Reference Guide
Some of the important features that the model incorporates are:
• The Frame Relay example model is developed in a modular fashion, with
simple interface requirements for DTE and DCE. The FRAD implementation takes care of interfacing the application to the Frame Relay network. Applications may be supported directly on top of the FRAD, or via an interface module if the applications relies on some other protocol. As an example, a Frame Relay-IP interface model (FRIPIF) is provided to illustrate interfacing Frame Relay with IP (an example showing how to set up
applications using TCP/IP to support data transfer over a Frame Relay network is also provided).
• Each PVC supports separate bi-directional specification of all the necessary
parameters associated with it (i.e., CIR, Bc and Be along with the source and destination application information). More details on this topic are provided in a later section of this document.
• The model implements congestion control mechanisms at the DTE on a per-
PVC basis by monitoring the data transfer in accordance with the
subscription time parameters (CIR, Bc and Be) for that PVC over a
measurement interval, Tc. Data in excess of the Bc specification is marked
DE, and data in excess of Be specification is discarded immediately.
• The model allows emulation of zero CIR specification. In this mode all the
frames generated at the source are marked Discard Eligible (DE).
• The model implements congestion notification at the DCE by monitoring
network resource utilization in terms of the buffer usage for the outgoing link supporting each PVC. If the buffer usage exceeds a certain preset threshold (to denote onset of congestion), the FECN and/or BECN bits are marked in the outgoing frames. The model allows the user to define the following thresholds (expressed as a percentage of the total bit capacity for that buffer) where congestion control mechanisms are affected:
— Enable Congestion Status — Disable network congestion
The model also implements congestion control at the DCE by discarding frames which are discard eligible (i.e., with their DE bit set), and in case of severe throttling discarding all the frames until the network recovers from the congestion status. Again, these mechanisms are activated when the buffer usage exceeds predefined thresholds. The following four attributes are used to set these thresholds:
— Start discarding DE frames — Start discarding all frames — Stop discarding all frames — Stop discarding DE frames
Reference Guide 1—Frame Relay
Additionally, the model provides an option for the network to participate in congestion control by setting the DE bit in frames if the network is in a congested state. This feature can be incorporated in the model by enabling the following parameter:
— Network action on DE bit
All of the seven parameters above are attributes of the Frame Relay switching node. The values for these parameters are computed as a percentage of the buffer capacity for that switching node.
• The model implements Frame Relay fragmentation. The Frame Relay layer
fragments application packets that are larger than a specified maximum frame size that the network can carry. At the destination end, these fragments are reassembled to form the original application data packet.
• The model calculates standard user-information-transfer performance
parameters like end-to-end delays, congestion status of the network with respect to time, and the frame residual error rate (RER). More details on these statistics can be found in a subsequent section of this document. Additionally, specific portions of the model can be easily modified to incorporate other custom statistics, if desired.
• The number of application processes running directly on top of the FRAD can
be easily changed without modifying any of the existing models. More details on this topic are provided in a later section of this document.
• The Frame Relay demand object provides you with a way to define Frame
Relay PVCs. You define the PVC as a demand object overlaid between two nodes in a network model, then you can define specific characteristics of the PVC on the Layer 2 Mapping compound attribute of the interface. You do not need to use the Frame Relay PVC Config node in the model.
• The Frame Relay PVC Config node allows you to define a traffic pattern and
assign it to a PVC. You can also create multiple PVCs for a given source- destination pair; this allows you to characterize different PVCs to support specific traffic patterns, thereby providing varying QoS for different higher- layer traffic flows.
• The standard model library includes a set of Frame Relay “cloud” models that
allow you to abstract parts of a network infrastructure. A cloud behaves like a simplified Frame Relay switch with many ports: it supports connectivity between attached devices, but does not include complex operations like buffer management. Clouds allow you to model a subnetwork’s packet-loss
1—Frame Relay Reference Guide
and latency characteristics while its inner workings remain transparent; see the Standard Model Library chapter of Modeling Concepts in the Modeler documentation or of the User Guide in the Guru documentation for more information.
Note—To add a Frame Relay demand object to your network model, open the
Frame Relay object palette and drag the “fr_pvc” object to the workspace.
Note—If you do not specify a name for the PVC, the default name is
Reference Guide 1—IGRP
IGRP
IGRP is the Interior Gateway Routing Protocol used in TCP/IP and OSI internets. It is classified under the interior gateway protocol (IGP) class of routing protocols, but can also be used as an exterior gateway protocol (EGP) for inter-domain routing.
Like the RIP routing protocol, IGRP uses distance vector routing technology. This technology specifies that gateways exchange routing information only with adjacent gateways. This routing information contains a summary of information about the rest of the network. The concept is that each router need not know all the router/link relationships for the entire network. Each router advertises destinations with a corresponding distance and the routers hearing the information adjust the distance and pass along the information to neighboring routers.
The distance information in IGRP is represented as a composite “metric” of available bandwidth, delay, load utilization, and link reliability. The dependence on all these metric-related parameters allows fine tuning of link characteristics to achieve optimal paths.
Model Features
The following features are implemented.
• The IGRP routing tables maintained at each node are initialized with the local
gateway’s IP addresses. The cost for these routes is set to 0. The cost computation for routes learned from neighboring routers is performed using topological delay, bandwidth of the narrowest bandwidth segment, channel occupancy of the route, and the route reliability.
• Equal-cost multi-path support: In many cases it makes sense to split traffic between two or more paths. IGRP will do this whenever two or more paths are equally good.
• A modified Bellman-Ford algorithm (as documented in the reference above)
is used during route computation.
• The start time at which the first regular routing update is generated is a
parameter that you can control on a per-node basis.
1—IGRP Reference Guide
• Holddown timer is implemented to avoid the problems due to potential latency in route propagation through the entire network. The holddown rule indicates that when a route is removed, no new route will be accepted for the same destination for some period of time. This holddown period ensures that triggered updates are delivered to all other gateways, so that any new routes received are indeed new, not an old route being re-inserted by a gateway that has not received the triggered update.
• Split horizon with route poisoning is implemented to avoid routing loops. Split horizon should prevent loops between adjacent gateways. Route poisoning is intended to break larger loops. The rule is that, when an update shows the metric for an existing route to have increased sufficiently, there is a loop. The route should be removed and put into holddown.
• IGRP simulation efficiency. With this technique (enabled by default), routing update messages are not sent after a certain point in time, resulting in faster simulation execution. If you know that routing information will not change after a certain point, enter that time as the IGRP Stop Time.
It is intended that only nodes that are gateways (those that have more than one network interface) implement IGRP. Therefore the IGRP routing tables are only used to route packets between gateways. This is important in order to reduce unnecessary simulation events and therefore improve simulation efficiency. The IGRP route metric computation requires particular units for the bandwidth and delay components. These values are specified for each media type in a common external file (igrp_metric_compute_support.gdf):
Figure 1-1 Units for Bandwidth and Delay Components
Reference Guide 1—IP (Internet Protocol)
IP (Internet Protocol)
The Internet Protocol (IP) is a connectionless network-level protocol that interconnects networks. In the IP model suite, IP services transport layer protocols such as TCP and UDP. As such, IP is situated beneath the transport layer. In turn, IP relies on data link layer technologies, such as Ethernet and Token Ring, to relay packets to other IP modules. This document describes key features of the IP model shipped as part of the standard OPNET model library.
1—IP (Internet Protocol) Reference Guide
Model Features
This section provides a list of the main features available in the Internet Protocol model:
• The IP model suite captures the following protocol behavior:
Table 1-7 IP Model Features (Part 1 of 2)
Feature Description
IP Addressing IP addresses are specified using dotted decimal notation (also
called dotted quad notation). The model incorporates
addresses from class A, B, C, and D types of addresses. Class D addresses are used for IP Multicasting, and is modeled in OPNET’s specialized IP Multicasting model.
Routing and Forwarding IP routers are modeled using a finite buffer with two available processing modes: Slot-based and Central.
Routing in the IP model is provided by:
• dynamic routing tables — created by dynamic routing protocols
• static routing tables — user-configured Fragmentation and
Reassembly
The IP model uses the same principles and methods as actual IP implementations to break up datagrams before transmission and to reassemble them at the destination IP module. This is essential if the model is to produce the same traffic
characteristics that occur in the modeled network. Fragmentation occurs when the length of a datagram
forwarded by an IP module exceeds the maximum transfer unit (MTU). Reassembly functions are performed on datagrams when they reach their final destination.
Reference Guide 1—IP (Internet Protocol)
Processing Delay and Queuing Capacity
The performance of IP-based networks depends on the characteristics and capabilities of the network’s switching elements. To support accurate studies involving these parameters, the IP model allows you to specify queuing capacities and packet processing speeds for each IP module.
Queuing Algorithms The model offers several queuing management options:
• a standard FIFO queue
• a weighted fair queue (WFQ) — Implemented as a bitwise round-robin fair queuing algorithm, it simulates the WFQ found in most routers.
• priority queuing (PQ) • custom queuing (CQ) • random early detection (RED)
• weighted random early detection (WRED)
IP Cloud Models The standard model library includes IP cloud models that allow you to abstract parts of a network’s infrastructure. A cloud behaves like a simplified IP-based router with many ports: it supports connectivity between attached devices, but does not include complex operations like WFQ. Clouds allow you to model only packet-loss and latency characteristics of a subnetwork. See for more information.
End of Table 1-7
Table 1-7 IP Model Features (Part 2 of 2)
1—IP (Internet Protocol) Reference Guide
Reference Documents
The model is based on information from the following sources:
IP Addressing
An IP address is a 32-bit number consisting of two fields: the network number (identifying the network) and the host number (identifying a particular host within the network). An IP address is specified using dotted decimal notation. This notation uses four decimal integers separated by decimal points, where each integer is the value of one octet (8 bits) of the IP address. For example, the following 32-bit address:
Thus, an IP address is represented using the E.F.G.H format, where E, F, G, and H are decimal numbers between 0 and 255. Each of these decimal numbers represents one byte of the IP address. Together, the four bytes (32 bits) represent the network and host address. However, which numbers refer to the network and which numbers refer to the host depends upon the Internet address class. The following illustration highlights the most-commonly used address classes:
Figure 1-2 Internet Protocol Address Classes Table 1-8 Reference Documents
RFC-791 Internet Protocol
Parekh, A.K.J, and Gallager, R.G.
A Generalized Processor Sharing Approach to Flow Control in Integrated Services Networks: the Single Node Case,
Laboratory for Information and Decision Systems, Massachusetts Institute of Technology, January 1991
End of Table 1-8 11000000 00001001 11001000 01101100 is written as: 192.9.200.108 7 bits 0 1 0 1 1 0 14 bits 21 bits 16 bits 8 bits Network Host Network Host Network Host Class A Class B Class C 24 bits
Reference Guide 1—IP (Internet Protocol)
You can differentiate among the IP address classes by looking at the three most significant bits of the address. For example, the range of values for the first byte of Class C addresses begins at 192 (in decimal notation). The following table lists the range of values available as valid addresses for Class A, B, and C networks, based on the reservation of bits for network and host identifiers.
An integral part of IP addressing is subnet addressing (also called subnet routing or subnetting). Subnetting allows one IP network address to define and identify multiple underlying physical networks. This feature helps solve the potential problem of running out of the finite number of available IP network addresses. Subnet routing is achieved using extra bits from the host portion of the IP address. For example, consider a class B IP address: since it has 16 host number bits, it can assign 65536 node numbers. However, network
configuration may require that nodes within this network are placed on different physical networks (for efficient administration, perhaps). If each physical network contains no more than 255 nodes, the higher order 8 bits of the host number portion of the IP address can be used for subnet addressing. Thus, the IP address representation changes from:
Figure 1-3 Internet Protocol Subnet Addressing Table 1-9 Internet Protocol Address Range
Class Size (bytes) Range of Values (dotted quad)
Network Host Network Portion Host Portion
A 1 3 1 – 126 0.0.1 – 255.255.254 B 2 2 128.1 – 191.254 0.1 – 255.254 C 3 1 192.0.1 – 223.255.254 1 – 254 End of Table 1-9 Network Host 8 16 24 8 16 24
to:
0 31 0 311—IP (Internet Protocol) Reference Guide
A subnet mask identifies how IP nodes interpret IP addresses. When
configuring a subnet mask, the number of extra bits used as the subnet number is determined. Then, a value of 1 is set for every bit position in the IP address that will be recognized as part of the network or subnetwork number.
Additionally, a value of 0 is set for every bit position in the IP address that should be recognized as the host identifier (or node number). For example, if the eight most significant bits from the host identifier field of a class B address are to be used as the subnet numbers, the resulting subnet mask will be 255.255.255.0. For more details on IP addressing or subnetting procedures, see any standard IP textbook, or to the Internet Engineering Task Force document RFC-950.
Reference Guide 1—IP Multicast
IP Multicast
IP Multicast is an efficient way to distribute information from a single source to multiple destinations. The model library includes a highly detailed model for simulating IP multicast networks. This document describes key features of the IP Multicast model shipped as part of the standard model library.
Model Features
The IP Multicast model supports the following protocol features.
Reference Documents
The model is based on information from the following sources:
Table 1-10 IP Multicast Model Features
Feature Description
Rendezvous Point (RP) Selection
The model supports dynamic and static RP selection. Two